Sliding bearings are used in internal combustion engines, for example as bearing lining shells and thrust flanges. Bearing lining shells for use as crankshaft journal bearings in internal combustion engines, are typically semi-cylindrical in form. Bearing lining shells are provided with one or more generally semi-annular thrust flanges (e.g. at each axial end of the bearing shell) before being assembled into the bearing of an engine.
The bearing surfaces of sliding bearings generally have a layered construction. The layered construction frequently comprises a strong backing material, such as steel, of a thickness in the region of about 1 mm or more; a layer of a first bearing material (the “lining layer”), such as a copper-based material (e.g. bronze) or aluminium-based material, is adhered to the backing, and of a thickness generally in the range from about 0.1 to 0.5 mm (e.g. 300 μm of copper-based alloy of 8% wt Sn, 1% wt Ni, and balance of Cu, apart from incidentally impurities); and a layer of a second bearing material (the “overlay layer”) of a metallic or polymer-based bearing material adhered to the surface of the lining layer and having a thickness of less than about 25 μm.
The surface of the second layer forms the actual running or sliding surface, which, in use, faces the surface of a co-operating shaft journal. The backing provides strength and resistance to deformation of the bearing shell when it is assembled in a main bearing housing or in a connecting rod big end, for example. The first layer may provide suitable bearing running properties, if the second layer should be worn through for any reason. As noted above, whilst the first bearing material provides seizure resistance and compatibility, it is generally harder than the material of the second layer. Thus, the first layer is commonly inferior to the second layer in terms of its ability to accommodate small misalignments between the bearing surface and the shaft journal (conformability) and in the ability to embed dirt particles circulating in the lubricating oil supply, so as to prevent scoring or damage to the journal surface by the debris (dirt embedability).
The first bearing material is commonly chosen from either an aluminium-based alloy (i.e. having no more than 25% wt additive elements, with the balance to 100% wt of aluminium) or a copper-based alloy material (i.e. having no more than 20% wt additive elements, with the balance to 100% wt of copper). Aluminium-based alloys generally comprise an aluminium alloy matrix having a second phase of a soft metal therein. Generally, the soft metal phase may be chosen from one or more of lead, tin and bismuth, however, lead is nowadays a non-preferred element due to its environmental disadvantages. Copper-based alloys such as copper-lead and leaded bronzes are also likely to fall into disfavour eventually due to these environmental considerations and may be replaced by lead-free copper alloys, for example.
The second bearing material layer, which forms a mating fit with the shaft journal with a clearance for lubricating fluid, is also known as an overlay layer and is formed of a matrix of plastics polymer material with filler, which for example has a thickness of 4 to 40 μm.
WO2004/1 13749 of common ownership herewith describes a plastics polymer-based bearing layer having a preferred conventional overlay layer thickness of 10 to 30 μm, when deposited upon a bearing having a layer of metallic bearing material, and which overlay is intended to last the life of the bearing. Moreover, the plastics polymer-based bearing material described in the document is also able to constitute a sole bearing layer when deposited directly upon a strong backing layer at a preferred thickness range of 40-70 μm. The plastics polymer-based overlay material comprises: a matrix of a polyimide/amide or modified epoxy resin and fillers selected from: 15-30% vol metal powder; 1-15% vol fluoropolymer; 0.5-20% vol ceramic powder; and 2-15% vol silica. Plastics polymer overlay layers based on such formulations exhibited high wear resistance and fatigue strength. However, the relatively high levels of filler content tended to make the overlay layer relatively hard and consequently less able to absorb and nullify the deleterious effects of debris particles circulating in the lubricating oil, such that the dirt embedability may be less than desired, which can lead to scoring of the bearing surface and/or the shaft journal surface. Accordingly, WO 2004/1 13749 also discloses that the polyimide/amide resin matrix also contains additions of vinyl resin to improve the conformability of the resulting bearing layer. However, the vinyl additions tend to weaken the polyimide/amide matrix in terms of strength.
WO2010/066396 of common ownership herewith describes a plastics polymer-based bearing layer comprising a matrix of a polyimide/amide plastics polymer material and having distributed throughout the matrix: from 5 to less than 15% vol of a metal powder; from 1 to 15% vol of a fluoropolymer, the balance being the polyimide/amide resin apart from incidental impurities (e.g. a layer of 12 μm thickness that comprises 12.5% vol Al, 5.7% vol PTFE particulate, 4.8% vol silane, <0.1% vol other components, and balance (approximately 77% vol) polyimide/amide). This provides a significantly higher proportion of an inherently stronger material than the plastics polymer-based bearing material of WO2004/1 13749. However, it remains desirable to further increase wear resistance and to further improve the fatigue strength of layers in bearing linings, particularly overlay layers.
A particular challenge to the performance of bearing lining layers and/or overlay layers is provided by the fuel-saving configuration of vehicle engines to “stop-start” operation, in which the engine is stopped and requires restarting each time the vehicle stops, in contrast to conventional engine operation, in which the engine is kept running throughout a vehicle's journey. Engines configured for stop-start operation may restart their engines about one hundred times more frequently than conventionally configured engines run continuously throughout each vehicle journey. Engine bearings are conventionally hydrodynamically lubricated, with little or no lubrication initially being provided to the bearings when the engine starts. Accordingly, stop-start operating of an engine can place increased demands upon the performance of the bearings.
It is also known to provide a plastics polymer-based bearing layer on top of a conventional metallic bearing alloy lining layer (e.g. with or without an intervening metallic overlay layer), as a so-called “bedding-in” layer that is intended to wear away as a sacrificial layer, leaving the conventional metallic bearing lining layer beneath as the running or sliding surface in the longer term. Such polymer bedding-in layers typically have relatively high contents of filler materials, generally comprising self-lubricating materials such as graphite, molybdenum disulphide and the like. As remarked above, high filler contents of inherently weak materials are detrimental to strength and wear resistance of the bearing layer, which consequently wears away relatively rapidly to fulfil the function of a bedding-in layer. Since the layer is intended to wear away relatively rapidly, it is generally relatively thin.